The invention relates generally to a method for phase locking and coherently combining multiple laser amplifiers, and in particular to sweeping amplifier phase modulators in small steps over a 2π range sequentially and selecting the phase of each that maximizes the combined output.
The intensity and, hence, the power available from a single-mode optical fiber is limited by either optical surface damage or nonlinear optical effects. These limitations can be overcome by coherent beam-combining of the output power from multiple optical fibers. Fine control of the optical phase is required for any multi-fiber approach.
The most relevant prior art is embodied in the three patents of Thomas M. Shay, 7,058,098 B1, 7,187,492 B1, and 7,233,433 B1. These three patents have similar titles: “Self-referenced (or Self-synchronous) Locking of Optical Coherence by Single-Detector Electronic-Frequency Tagging”. They employ the same basic principle of frequency tagging, and phase detection and processing. A simplified block diagram typical of this method is shown in FIG. 1. A master oscillator 1 is power divided 2 and provides the inputs to the N amplifiers 5 after first passing through modulators 3 and adjustable phase shifters 4. The method is based on tagging the N amplifiers 5 with a unique frequency. The modulation process produces unique side-band frequencies on the signal related to the tag frequency. The tag frequency sideband provides the means for identifying the signal of each specific amplifier output and in addition carries the phase information of the amplifier, which is necessary during the processing function of the method. This modulation is accomplished by the Tag Oscillator 12 modulating the amplifier input via the modulator 3. The type of modulation may be phase modulation, frequency modulation or amplitude modulation, anyone of which can serve the purpose of the method.
These three patents relate to the different types or combinations of types of modulation; however, the basic overall method is the same. The outputs of the amplifiers 5 are combined into a single output beam 10 by the coherent power combiner 6. A small sample of the output beam 10 is sampled by the coupler 7 which is input to the photodetector 8. The photodetector 8 output contains all of the individual amplifier outputs including the tagging sidebands. This complex output is mixed individually 11 with each tag oscillator 12. The mixer 11 output is processed 13 to extract the phase of the corresponding amplifier 5. The processor 13 then generates an error signal that produces the drive signal for the phase shifter 4 based on a negative feedback process. The negative feedback process is so configured as to correct the phase of each amplifier 5 output to match the phase of the other amplifiers and thereby provide the necessary condition to combine the power from each amplifier into a single coherent beam 10.